Pestology Blog
Bed Bug Fumigation, Spider Moms, and Fire Ant Viruses
Fairfax, VA – December 1, 2023
NPMA BUGBYTES is back! We return with two new co-hosts and NPMA Technical Team members Laura Rosenwald and Ellie Lane to share some highlights from our own graduate research.
Featured Article Summaries
Tawny Crazy Ant
The Thermal Breadth of Nylanderia fulva (Hymenoptera: Formicidae) Is Narrower Than That of Solenopsis invicta at Three Thermal Ramping Rates: 1.0, 0.12, and 0.06°C min −1
My doctorate research mostly focused on the invasive ant, Nylanderia fulva, also known as the tawny crazy ant (TCA). This species is native to South America where it isn’t widely considered a major pest in its native range. But, in the southeastern US where this species has been introduced, the absence of predators and lack of other natural factors to keep populations in check have allowed TCA colonies to dominate in some areas, reaching plague-like numbers where workers literally blanket the ground.
Once colonies reach these population numbers, they become nearly impossible to control with conventional methods. Dead and dying workers can even pile up in the thousands and completely cover treated areas, becoming a sort of corpse-bridge for other foragers to safely cross without ever coming in contact with a treated surface.
The overarching goal of my research was to investigate how the TCA was able to gain a competitive advantage over ant species and potential predators to become such a dominant force in a non-native environment. The hope was that this information could be used to improve our control efforts by gaining insight into the seasonal ecology of this invasive ant.
One surprising behavior being observed in the field at the time was that TCA workers were outcompeting and displacing a larger and more aggressive invasive ant in the southeastern US, the red imported fire ant (RIFA). What made this such an incredible feat is that the RIFA is widely considered one of the most aggressive and damaging invasive ants in the world. This was pretty remarkable because, up until this point, there really weren’t any other ants (invasive or native) that could put up much of a fight against the RIFA.
Armed with some interesting field observations, anecdotal evidence of aggressive displacement, and some background history on this newer invasive ant, I set out to explore the factors that could contribute to the TCA’s competitive success in the field. To do this, I laid out a series of experiments that focused on different aspects of competitive advantage such as body size and thermal tolerance. Or, an organisms ability to tolerate hot and cold temperatures. One of my first experiments, and the focus of this blog, was to investigate how a smaller ant species like the tawny crazy ant was able to find success against its larger competitor. To do this, I looked at how both species handled the cold and the heat.
Both the TCA and RIFA are from South America where temperatures are generally warmer than what they experience in many parts of the Southeastern US. Being able to handle the cold or heat better than your competitors is a major advantage in the ant world. The hot and cold temperature limits of an organism are known in ecology as the critical thermal minimia (CTmin) and critical thermal maxima (CTmax). If you are more cold-tolerant than other ant species in your area, then you can start looking for food sooner in the morning or later into the colder months when temperatures may be too cold for your rivals to endure. And, the same applies for tolerance to hotter temperatures. My goal was to test the cold and heat tolerance of both species to determine who had the advantage here. My hypothesis was that the RIFA, the larger bodied ant, would be more tolerant to both temperature extremes.
To find out if temperature tolerance could be what was giving the TCA a competitive advantage, I exposed workers of both species to increasing or decreasing temperatures and recorded when workers of each species would reach their limit. By that, I mean I recorded when the ants would collapse and not move. Importantly, I also tested their temperature tolerance at three different rates of temperature change, meaning that I changed how fast or slow the temperature would change. This gave me a more accurate picture of not only what the thermal limits of each species were, but also how well they tolerated different rates of temperature change. This allowed me to better estimate how each species may respond to more ecologically relevant conditions that would be closer to what we see in nature without actually running experiments outside.
What I found wasn’t exactly surprising. The TCA had a narrower thermal tolerance range than the RIFA, meaning that RIFA workers were able to withstand both colder and hotter temperatures than the TCA. This wasn’t surprising because RIFA workers are larger on average than TCA workers.
So, the outcome of the study didn’t point to TCA being a better competitor for food at any thermal extremes. Later studies completed by other researchers working on this species determined that the TCA actually has the ability to detoxify the RIFA’s venom when workers fight. While there is still a wide gap in knowledge surrounding the TCA and why it’s so successful in the US, the data we have so far points to sheer numbers, high reproductive rates, and escape from natural predators as keys to success. Though, our data on temperature tolerance does help to shed more light on the seasonality of this species, giving PMPs more information on when control measures should begin each year to help gain the upper hand on this species.
Article by Mike Bentley, PhD, BCE
References
M. T. Bentley, D. A. Hahn, F. M. Oi, The Thermal Breadth of Nylanderia fulva (Hymenoptera: Formicidae) Is Narrower Than That of Solenopsis invicta at Three Thermal Ramping Rates: 1.0, 0.12, and 0.06°C min −1, Environmental Entomology, Volume 45, Issue 4, August 2016, Pages 1058–1062, https://doi.org/10.1093/ee/nvw050
Red Headed Flea Beetles
Temporal and Spatial Factors Influencing Systena frontalis (Coleoptera: Chrysomelidae) Behavior in Virginia Nurseries
The subject of my study was the red-headed flea beetle which requires some introduction to anyone who is not in agriculture or ornamental production. This beetle causes damage by eating the leaves of a wide variety of plants. In the context of my study, I was concerned about the damage this beetle can do to ornamental plants such as hydrangeas, hollies, and sweetspires in commercial nurseries, specifically in eastern Virginia. The damage it causes is of top concern to growers who value and require their crops to be of high quality.
This pest is likely a native species that has grown to be a problem within the last decade, being cited by Virginia nursery growers as their top pest of concern. The thing is that there isn’t much known about this pest the way there is about other common agricultural pests that have decades of research.
In efforts to control the beetles, growers spray products as often as three times a week when beetle numbers are high. This brings me to one of the study objectives which was determining when the beetle numbers are high. Different climates affect their emergence and the numbers of generations that appear per year, and knowing when this is happening allows the growers to use certain products that have limits to the number of uses per year when they will be most effective.
What I did was go out to two commercial nurseries in eastern Virginia and every week from about March to October, for two years. My lab and I vacuumed sections of the nursery to count how many beetles per plant were present. We looked at three different plants that were known to have heavy beetle damage specifically, Hydrangea paniculatas in the variety limelight.
We found that there were likely three generations a year with peaks in adult numbers occurring in June, late July, and late August. Each beetle life cycle takes about a month and at the end of the season, the eggs in the soil will overwinter to the next year. The larvae live in plant soil and feed on the roots but don’t cause visual damage.
The other major section of this was to find when during the day the beetles were most active so that growers might time their spraying to most effectively target the beetles. I went out and counted how many beetles were doing what every 2 hours for a full 24 hours and found them to be active during the day especially between 11am-3pm.
The last section of this paper looked at how many beetles caused what level of damage and to do this we put 0, 5, and 25 beetles in a cage with one hydrangea for 1 week and then assessing damage to leaves.
Overall, we found that 5 individuals which according to the population studies, was a high density, can cause up to 4% damage which is more than you think. Overall, this study provided information so growers now know when to expect high numbers of beetles, when during the day they are most active, and how much damage different numbers of beetles can cause so they can determine what their action threshold may be.
There were so many moving parts to this project that worked together to tell the story of what the beetles were up to and what we could learn about them. There was even another section of the project that looked at a chemical attractant as a potential for a lure for the beetles but it was unsuccessful under the available conditions.
There is still research that will be needed to bridge the remaining gaps such as if spraying at certain times of the day is more effective. But overall, that gives you a general idea of what I was up to for two years before starting at NPMA.
Article by Ellie Lane
References
Lane, Eleanor L., and Alejandro I. Del Pozo-Valdivia. "Temporal and spatial factors influencing Systena frontalis (Coleoptera: Chrysomelidae) behavior in Virginia nurseries." Environmental Entomology 52.4 (2023): 730-739.
Lane, Eleanor L., and Alejandro I. Del Pozo-Valdivia. "Red Headed Flea Beetle in Virginia Nurseries." Virginia Cooperative Extension ENTO-464 (2021). https://www.pubs.ext.vt.edu/ENTO/ENTO-464/ENTO-464.html
Spider Bacteria
Endosymbiotic Rickettsiella Causes Cytoplasmic Incompatibility in a Spider Host
When someone looks at a spider, I’m sure most of people don’t immediately think “I wonder what bacteria live inside that spider?”
This blog post is a confession: I do. This is what graduate school does to you.
My work in graduate school focused on bacteria that live inside arthropods called endosymbionts. These endosymbionts can influence their arthropod hosts in a number of ways and can range from beneficial, detrimental, or neutral. Most of these endosymbionts in arthropods are passed on through the mom- meaning that whatever she happens to be infected with, her offspring will be infected with too. Some endosymbionts have a particular talent for ensuring that they get passed on to the next generation by manipulating their hosts reproductive systems.
There are a couple of ways that endosymbionts have been documented doing these reproductive manipulations through a wide variety of arthropods. One of these ways is a process called cytoplasmic incompatibility, or CI. By using this cytoplasmic incompatibility, the bacteria are able to pick and choose when a host is able to successfully reproduce. The bacteria do so by manipulating the sperm in the infected male. If that infected male happens to mate with an uninfected female, the sperm is unable to be corrected, and the eggs in the female fail to be fertilized. Remember that these bacteria are passed on through the mom, so if the mother arthropod is not infected with the bacteria, they won’t make it to the next generation. Therefore, these bacteria are often the puppeteers pulling the strings on whether or not offspring will be produced from a mating.
It turns out that some of the arthropods with the highest diversity of these endosymbionts that have the potential to reproductively manipulate their hosts are spiders (White et al. 2020). In particular, I studied these endosymbionts in a sheet-weaving spider known as Mermessus fradeorum. These spiders are found everywhere, but are commonly found in agricultural areas.
I collected these spiders from an alfalfa field and brought them into the lab. We first asked what they happened to be infected with and determined that they were able to be infected with up to five different potential reproductive manipulators, and every spider was infected with at least one of these endosymbionts. Specifically, we found that these spiders were infected with a strain (or variety) of Rickettsia, three strains of Wolbachia, and a strain of Rickettsiella. This strain of Rickettsiella infected nearly every single spider in our sampled population, so we decided to see if it was causing a reproductive manipulation in M. fradeorum.
To do so, we first took some of our spiders and treated them with antibiotics to clear them of their endosymbionts. We used the offspring of those antibiotically treated spiders to account for any effects that may have occurred through the antibiotic treatment. Using these endosymbiont-free spiders, we then created the crosses that we would need to document the occurrence of cytoplasmic incompatibility being caused by Rickettsiella.
Following a lot of romancing by the spiders, we finally had the results of our crosses. Overall, there was no difference in the number of eggs laid across the four crosses, meaning that all the females were still producing the same number of eggs despite their infection status. We found that infected females mating with infected males had 99.7% of their eggs hatch, while uninfected females mating with uninfected males had 83.9% of their eggs hatch. Infected females mating with uninfected males had 91.3% of their eggs hatch. However, uninfected females mating with infected males only had 13.2% of their eggs hatch. This meant that Rickettsiella caused cytoplasmic incompatibility within Mermessus fradeorum.
This documentation of Rickettsiella causing cytoplasmic incompatibility in M. fradeorum emphasizes the importance of endosymbionts and their relevance to their arthropod hosts. By examining a small, brown, agricultural spider, we were able to find a bacteria that is capable of doing manipulations that we could potentially use for pest management. Currently, there are a few companies that are using Wolbachia and its cytoplasmic incompatibility for local population control in mosquitoes. Our study emphasizes that these potential tools may be much more widespread in arthropods than previously appreciated, and that they could potentially be used in a broader spectrum than just mosquitoes.
Maybe next time you take a look at a spider, you’ll also think about the bacteria hiding away inside. I saved you from the struggles of graduate school. You’re welcome.
Article by Laura Rosenwald, BCE
References
Rosenwald L.C., Sitvarin M.I., White J.A. 2020 Endosymbiotic Rickettsiella causes cytoplasmic incompatibility in a spider host. Proc. R. Soc. B 287: 20201107. http://dx.doi.org/10.1098/rspb.2020.1107
White, J.A., Styer, A., Rosenwald, L.C. et al. Endosymbiotic Bacteria Are Prevalent and Diverse in Agricultural Spiders. Microb Ecol 79, 472–481 (2020). https://doi.org/10.1007/s00248-019-01411-w
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